The rotary power transmission device having an input shaft, an output shaft, and a fluid torque converting structure which includes a pump impeller, a turbine member, and a stator member incorporated between the input shaft and the output shaft and which further incorporates a lock up clutch for selectively mechanically coupling the input shaft and the output shaft, is already well known as a device to be incorporated in the power transmission system of an automobile. Many of these rotary power transmission devices further incorporate rotary shock absorbers in their rotary power transmission routes which provide flexibility in the rotational direction between the rotary power transmission members of their lock up clutches, so as to avoid transmission of shocks between their power input shafts and their power output shafts, i.e. between the engines and the driven wheels of the automobiles to which these transmission devices are fitted. A rotary shock absorber of this kind comprises first and second disk elements arranged as spaced along their common axis and connected with each other with respect to their rotational movement, a third disk element provided between the first and the second disk elements so as to be rotatable relative to the first and second disk elements, and a plurality of compression coil springs adapted to act between the combination of the first and second disk elements and the third disk element.
Conventionally, a rotary shock absorber of this type has generally been constructed with such a structure that the compression coil springs are arranged along a circle having a relatively small diameter, so that the rotary shock absorber is arranged along the root portion of the pump impeller. However, in order to meet with the requirement that the fluid torque converter and the lock up clutch should be mounted within a very limited axial space such as is available in the case of a front engine front wheel drive automobile or a so called FF automobile, it has been proposed to provide such a rotary shock absorber in an annular space left between the outer peripheral portions of the fluid torque converter generally having a toroidal shape and the lock up clutch generally having a disk shape, so that the compression coil springs which have a relatively large cross sectional area can be fitted into the annular space having an approximately triangular cross section. In this case, the rotary shock absorber is constructed to be of a relatively large diameter.
When a rotary shock absorber of the abovementioned type is constructed to have a relatively large diameter, two of the above mentioned first, second, and third disk elements inevitably become ring shaped elements having relatively small radial widths as compared with their radiuses or diameters, while the third one inevitably becomes a substantially disk shaped element having a relatively large diameter. When the disk element having such a relatively large diameter is positioned so as substantially to traverse the space left between the fluid torque converter and the lock up clutch, said lock up clutch typically being engaged and disengaged according to the direction of flow of the hydraulic fluid being supplied to the torque converter, a substantial pressure difference is liable to be generated between the opposite sides of the disk element which is to be made of a relatively thin plate member in order to reduce the weight of the rotary shock absorber, and thereby the disk element is liable to be irregularly deformed.